﻿136 Prof. E. Rutherford on the Retardation of the 



small piece of photographic plate. By means of side screws, 

 the distance of the slit and photographic plate from the source 

 could be altered at will. Over this apparatus was placed a 

 brass tube P closed at the upper end. To the lower end was 

 attached a flange FF, which was ground accurately to fit the 

 base-plate. By the use of tap-grease the flange formed an 

 air-tight joint, and the vessel could be at once exhausted by 

 a Fleuss pump without the necessity of waxing down the 

 brass tube to the base-plate. This saving of time as well as 

 of trouble was of great importance in experiments to be 

 described later, in ^hich the magnetic deflexion of the a rays 

 from radium A was determined. 



The whole apparatus was fixed on a wooden frame and 

 placed between the poles of a large electromagnet so that the 

 source of rays and the slit were parallel to the direction of 

 the magnetic field. In these experiments, the magnetic field 

 was practically uniform over the whole path of the rays. The 

 extent of this magnetic field is shown by the dotted lines LL 

 in the figure. The electromagnet was excited by a constant 

 current of 12 amperes, and was kept from overheating by an 

 electric fan. The duration of exposure of the plate in the 

 magnetic field was usually two hours, the direction of the 

 field being reversed every ten minutes. 



In most of the experiments, the region between the active 

 source and the photographic plate was divided into two equal 

 parts by means of vertical mica plates. One half of the 

 active wire was either bare or covered with some known 

 absorbing screen, while the other half of the wire was 

 covered with the number of layers of aluminium-foil under 

 examination. In this way two sets of bands were obtained 

 on the photographic plate, the distance between the centres 

 of the bands of each set representing twice the deflexion in 

 the magnetic field of the pencil of rays from the normal. The 

 distance between the deflected bands is inversely proportional 

 to the velocity of the a particles after emerging from the 

 absorbing screen. 



Measurement of the Photographic Plates. 



In a previous paper, I pointed out a peculiarity exhibited 

 by the photographic trace of a pencil of a rays from the 

 active wire when a narrow slit was used. The trace of the 

 pencil of rays consisted of two dark lines with sharply 

 defined outside edges, and with a whitish line running sym- 

 metrically down the centre of the trace. This effect was 

 shown to be a necessary consequence of the fact that the 



